The usage is the same if you send to someone sitting next to you or to someone across the planet.
To give you an idea of usage, I use BBM a LOT. I have about 40 people in my BBM contact list and have 2 or 3 group convos going at the same time, as well as a few individual conversations. I also get a lot of email, surf the web, and stream music with Pandora. My usage for last week was 430M. My plan is 'unlimited' but is really capped at 5G, so I'm nowhere near going over.
My son uses his even more for internet, especially videos, and last month used 1.9G. Still no extra fees.
